This invention relates to the field of geophysical prospecting. More particularly, the invention pertains to source waveforms for use in electroseismic exploration.
The electroseismic method is a geophysical prospecting tool aimed at creating images of subsurface formations using conversions between electromagnetic and seismic energy. The electroseismic method is described in U.S. Pat. No. 5,877,995 (Thompson, et al.). The essence of the electroseismic method is that high levels of electrical energy are transmitted into the ground at or near the surface, and the electrical energy is converted to seismic energy by the interaction of underground fluids, including hydrocarbons, with the rock matrix. The seismic waves are detected at or near the surface by seismic receivers. To be effective, this method requires an input current source with the following characteristics:
The source should produce large current levels over extended time.
The source should have high electrical efficiency.
The source should contain little or no DC to avoid plating the electrode array.
The frequency content of the source should be adequate for the exploration needs.
The correlation of the source waveform with its reference should have sufficiently low side lobe levels.
Little has been published to date on electroseismic waveforms because of the newness of the technique. However, in conventional seismic exploration, a seismic vibrator is sometimes used as an energy source to generate a controlled wavetrain (known as a sweep) which is injected into the earth. When the resulting recorded seismic data are correlated with the sweep wavetrain or other reference, the correlated record resembles a conventional seismic record such as that which results from an impulsive source.
When a source waveform is correlated with its associated reference, there will typically be a large peak at the onset time of the waveform surrounded by lower peaks at earlier and later times. These lower peaks are the correlation side lobes. Correlation side lobes are undesirable because they can mask smaller desired seismic returns.
It should be noted that the source waveform is just one piece of the electroseismic system. Other factors of importance include the power waveform synthesizer (that creates the waveform) as well as the input electrode array, the seismic receiver arrays and various field implementation issues.
As stated above, there is little current technology on electroseismic waveforms because of the newness of the technique. Some obvious approaches might include pulsing or pseudo-random square-wave sequences. Repeated pulses are inefficient (in energy/second) compared to continuous waveforms. Square-wave waveforms would be expensive to implement at the required high current levels and would also dissipate energy in unwanted high-frequency components.
What is needed is a source waveform that satisfies the requirements stated above. The present invention satisfies this need.
In one embodiment, the present invention is a method for electroseismic prospecting comprising the steps of (a) selecting a source waveform and corresponding reference waveform, both chosen to reduce correlation side lobe amplitudes, (b) generating an electrical signal based on the source waveform, (c) transmitting the electrical signal into the ground, (d) detecting and recording the seismic signals resulting from conversion of the electrical signal to seismic energy in subterranean formations, and (e) correlating the resulting seismic signals with a reference waveform to produce a correlated seismic record. Preferably, the reference waveform is chosen to substantially minimize side lobes when correlated with the particular source waveform used.
The source waveform may be constructed from individual cycles of a 60 cycles/sec (Hz) sine wave, i.e., standard AC electrical power, with the polarity of some such cycles inverted as governed by a binary sequence code. The binary code is selected to custom design an extended, but finite, source wave that has a reference wave that substantially minimizes correlation side lobes when the source wave and the reference wave are correlated together. The reference wave may be the source wave itself or a waveform derived from the source wave. Where deeper penetration of the subsurface is desired, another embodiment of the present invention constructs frequencies lower than 60 Hz by switching between the three phases of a 3-phase power source.
In some embodiments of the invention, the binary sequence used is a maximal length shift-register sequence, and circular correlation (defined below) is used for the last step. In other embodiments, two source waves are transmitted into the ground. One is a 60 Hz sinusoid wave element sequenced by one member of a Golay complementary sequence pair; the other is the same wave element sequenced by the other Golay pair member. The resulting seismic returns are separately correlated with their respective input waves and then summed. Both the Golay sequences and the maximal length shift-register sequence have excellent correlation side lobe reduction properties, with the side lobes being theoretically reduced to zero in the case of the Golay sequences.
According to the present invention, side lobes may be further reduced, for any pseudo-random binary sequence-generated source waveform, by making the source wave as long as possible before recording equipment limitations require that the source generation must be interrupted.